1. Field of the Invention
The present invention relates to an ink jet recording apparatus. More particularly, the invention relates to an ink jet recording apparatus of the on-demand type where characters and images are recorded by discharging ink only when recording is needed. Also, the present invention is not only applicable to the printing on paper sheets used in the office, but also, applicable to the industrial apparatus that records on all the media serving as ink supporting elements that accept the provision of ink, such as cloths, threads, sheets, among some others.
2. Related Background Art
The ink jet recording method, in which recording is made by discharging a desired liquid by means of bubbles created by the application of thermal energy that acts upon the liquid has excellent advantages that by use of a smaller apparatus, high resolution images can be recorded in colors at high speeds with a lesser amount of noises. Therefore, in recent years, the ink jet recording method has been widely utilized for a printer, a copying machine, a facsimile equipment, and many other office equipment. Further, this method has begun to be used for a textile printing system and other systems for the industrial use.
Along with the wider utilization of the ink jet recording technologies and techniques for the products in many fields, there are more demands in the provision of higher gradation, and higher image quality as well.
As one of the methods to materialize the higher gradation and higher image quality, there is a dither method and other pseudo multi-value recording methods. The recording head that adopts any one of these methods has a high nozzle density with a smaller volume of each droplet so as to form an image with more numbers of dots. However, with such method, the discharge frequency of droplets per recording sheet should be increased. As a result, the life of head becomes shorter. Also, with the higher density of nozzle of the recording head, there is a problem, among some others, that the costs of head manufacture are increased accordingly.
Now, therefore, there has been proposed a structure in which two or more electrothermal converting elements are provided for one nozzle each as disclosed in the specifications of Japanese Patent Laid-Open Application No. 55-132259, and Japanese Patent Laid-Open Application No. 08-332727. More specifically, with the two electrothermal converting elements arranged for one nozzle, two of them are driven at a time to obtain a droplet having a larger discharge amount (a large droplet), while driving either one of the electrothermal converting elements to obtain a droplet having a smaller discharge amount (a small droplet), thus changing the amounts of discharges. In this way, without changing the nozzle density from those conventionally in use, the amount of discharges can be changed with an extremely simple structure for the implementation of the higher gradation and higher image quality.
For the recording head that adopts the method in which a plurality of electrothermal converting elements are arranged for one nozzle, and the driving modes are changed in accordance with the amount of liquid to be discharged, it is still possible to utilize the conventional apparatus for manufacturing the recording heads for the implementation of the lower cost production.
However, in addition to the demands on still higher gradation and image quality as described above, there is a demand on the further enhancement of printing speeds of the ink jet recording method. In order to print at higher speeds, the electrothermal converting elements should be driven at higher frequency.
Here, one of the factors that may hinder the higher speed printing is the temperature rise of the head. For an ink jet recording head, approximately a 30% of given energy is used for discharging ink, but almost the entire remainders are changed into thermal energy to cause the head temperature to rise eventually. As a result, the higher the head driving, the more the head temperature rises. This may cause the instability of the discharge condition of droplets.
Now, in this respect, a method has been proposed in which the thickness of the protection film of the electrothermal converting element is made thinner so that the rise of the heat temperature is suppressed, while it is made possible to improve the foaming efficiency. FIG. 10A is a plan view which illustrates the method thus proposed. In FIG. 10A, an electrothermal converting element 53 is arranged in the nozzle 109. Also, FIG. 10B is a cross-sectional view which schematically shows the structure of the electrothermal converting element, taken along line 10B --10B in FIG. 10A. In FIG. 10B, a reference numeral 71 designates a silicon substrate on which are arranged among some others, the resistance layer 72 formed by HfB2 or other resistance material; the AL wiring layer 73; the lower layer 75 of the protection film formed by PSG or other insulation material; and the upper layer 76 of the protection film formed by SiO.sub.2 or other insulation material. Only the portion of the electrothermal converting element of the lower layer 75 of the protection film is removed by means of etching so as to make the protection layer thinner by 0.6 .mu.m corresponding to the thickness of the lower layer 75 of the protection film. In this way, the heat transferability becomes better so as to enhance the foaming efficiency. With the structure described above, the amount of energy that changes into heat is absorbed by the protection film, thus suppressing the temperature rise of the recording head.
Meanwhile, the major factor other than the thermal characteristics is the time required for refilling liquid from the rear end of the nozzle in an amount equivalent to the liquid droplet that has been discharged from the discharge port. Particularly, for the head capable of modulating discharge amounts, which is structured with two electrothermal converting elements in one nozzle, it is an important key to the attainment of the higher printing that the refilling time of the larger droplet should be made shorter rather than dealing with that of the smaller droplet. In consideration of the variation of discharge amounts, it is desirable to make the amount of the smaller droplet as smaller as possible with respect to that of the larger droplet in practical use (for example, a smaller droplet is 10 to 15 pl against a larger droplet of 40 pl) for the purpose of improving the gradation. Naturally, therefore, the amount of liquid that should be refilled is smaller for the smaller droplet as compared with the case where the amount equivalent to the larger droplet should be refilled.
Now, the inventors hereof have given attention to the positions of the two electrothermal converting elements which are arranged centering on the foaming of the larger droplets, and then, devised the invention taken out herein so as to attempt shorting the refilling time, while maintaining the freedom of nozzle designs to make the conventional nozzle manufacturing apparatus still applicable to the manufacture of new heads.